Tuning piston for waveguides



Nov. 7, 1961 H. J. ROWLAND TUNING PISTON FOR WAVEGUIDES Filed March 5, 1959 uiiorneys Hbwaad 1.120111% by dim;

guide section at some selected United States Patent 3,008,105 TUNING PISTON FOR WAVEGUIDES Howard J. Rowland, Egypt, Mass., assignor to D. S. Kennedy & Co., a corporation of Massachusetts Filed Mar. 5, 1959, Ser. No. 797,394 3 Claims. (Cl. 333-98) This invention relates to guided electromagnetic energy and more particularly to devices known as pistons or plungers used for tuning waveguide circuit elements at ultra high and microwave frequencies.

Waveguide systems are tuned by varying the effective length of a given waveguide section. The technique employed for varying effective length is to create a short circuit point at some location in the waveguide. The general practice is to use various types of terminating pistons within a waveguide having a shape conforming substantially to the cross-sectional area of the particular waveguide. The piston may have a shorting element integral with the piston or may comprise a mechanical guide bulkhead in contact with the waveguide inner faces which in turn carries the shorting section in front clear of the waveguide faces. The piston is moved axially through the waveguide by a suitable longitudinal screw; the shorting element effectively terminating the waveposition corresponding to the desired electrical effects.

In the past, it has been found difficult to make such tuning pistons fit closely against the inner walls of the waveguide. Because the travel of the piston is guided by the interior sides of the waveguide which generally are not smooth, the mechanical fit and electrical contact at various locations is often erratic and unpredictable. In many waveguide systems the terminating piston carries an appreciable current particularly where a large power output is involved; an imperfectly positioned piston will introduce serious power losses through wave reflections or standing waves arising from faulty electrical contact between the piston and the interior faces of the waveguide. Equally disadvantageous is the not infrequent occurrence of unwanted or spurious oscillations occurring in the waveguide sect-ion behind the shorting element of the piston assembly. The more imperfect the short effected by the piston the more likely the occurrence of troublesome oscillations and power losses in the waveguide section behind the tuning piston. This is sometimes called resonance in the back cavity.

Heretofore, in order to overcome the unpredictable deformation of the interior of waveguides and to make consistent contact, heavy, machined, complex pistons were employed. In relation to the section of waveguide in which it was positioned the piston had a far greater mass and itself tended to deform the waveguide at that location. Moreover, such heavy pistons frequently required nylon buttons serving as runners to facilitate smooth travel of the piston within the walls of the wave guide. No means of dealing with back cavity resonance was provided. Detuning of the affected wageguide section and returning elsewhere was suggested.

It is an object of this invention, therefore, to provide a shorting piston which makes good electrical contact with the interior faces of the waveguide at all points along its predetermined path of travel. Another object of the invention is to provide a piston having an operative shorting element which is always substantial normal to the direction of the guided energy. An additional object of the invention is to provide a shorting piston which readily conforms to the erratic surfaces found in most waveguide sections. A further object of the invention is to provide a piston which is simple, light in weight, readily fabricated and inexpensive. Still another object of the 3,008,105 Patented Nov. 7, 1961 invention is to provide a shorting piston which can effectively prevent or significantly attenuate oscillations which oftimes occur in a waveguide after the shorting point. Still another object of the invention is to provide a piston which can be easily positioned without need of complex or expensive axial screw means.

In the accomplishment of these and other objects of my invention I provide a piston fabricated entirely of thin but rigid sheet conductive material. A feature of the invention is the use of a guide bulkhead for the piston con sisting simply of a flat metallic plate; the broad and narrow edges of the plate are folded back at right angles to the plane of the plate to form a shallow vertical tray, the base having the approximate cross-section of the waveguide.

'Another feature of the invention is the provision of small spring finger contact elements formed integrally from a single strip secured to the top and bottom edges of the guide bulkhead compressably contacting the interior top and bottom broad faces of the waveguide.

An additional feature of the invention is the means used to position the main bulkhead from the waveguide narrow faces. A slight concave angular bend is made in the narrow edge flanges of the guide bulkhead member to provide effective springing conformation against the narrow side walls of the waveguide.

Another feature of the invention is that the electrically functional shorting element consists of a U-shaped section, the base of the U normal to the direction of the guided waves having the cross-sectional configuration of the waveguide with the arms of the U parallel to the broad faces of the guide. This element is supported away from the main structural element, the guide bulkhead already mentioned, by a simple horizontal metal plate.

At the back face of the guide bulkhead, two vertical plates parallel to the narrow waveguide faces are so attached to divide the broad dimension of the waveguide into three sections. A feature of this construction is the elfective division of the waveguide into three smaller sections. It is clear that electromagnetic energy of the propagated frequency which escapes beyond the shorting point cannot sustain oscillations behind the guide bulkhead. These three sections present a high impedance to the operating frequency thereby denying the back cavity to oscillations of any significant magnitude. Still another feature of the invention is the further function of the aforesaid vertical oscillation suppression plates in guiding the piston and providing contact with the broad faces. Small finger-shaped springs formed from single spring metal strips are secured at the top and bottom edges of the vertical plates and are compressed by the broad surfaces of the guide wherever the piston may happen to be positioned. A further feature of the invention is the use of the arms of a vertically disposed square U-shaped member for the oscillation suppression plates. The tops of the arms are bent over to make flanges to conveniently secure the member to the back of the guide bulkhead. The base of the U with an aperture at the center serves as a guide or connection point for associated axial locating means. The base also makes the entire piston structure more structurally stab-1e.

These and other objects and features of my invention will best be understood from a more detailed description of a preferred embodiment in which:

FIG. 1 is -a rear view of a piston embodying the present invention with the outlet of an associated waveguide;

FIG. 2 is a top view of the piston inside the waveguide shown in phantom; and

FIG. 3 is a side view of the piston inside the waveguide shown in phantom.

A shorting piston generally designated at 2 is shown in FIG. 1 within a waveguide 4, the waveguide having broad faces 6 and narrow faces 8. The principal mechanical functions of the invention herein are performed by a guide bulkhead 10 formed from a single piece of reasonably rigid conductive material. The corners thereof are cut out and a portion of broad edges 12 and narrow edges 14 corresponding to the broad and narrow faces 6 and 8 of the associated waveguide 4 are bent back at right angles forming top and bottom broad flanges 16 and side flanges 18 respectively. The result has the ap pearance of a shallow open box with a base 20, the flanges 16 and 18 being the sides thereof, the box disposed vertically within the waveguide. The base 20 of the box has a configuration and size similar to the crosssection of the associated waveguide.

Riveted to the outside of each of the broad edge flanges 16 is a strip 22 having two rows of small spring fingers 24 formed from a single piece of appropriate stock. As best shown in FIG. 2, the narrow edge flanges 18 of the guide bulkhead 10 are given a slightly bent shape presenting a convex angle tothe corresponding narrow waveguide face 8.

When inserted into the interior of a waveguide the vertical dimension of base 20 of bulkhead 10 is such that the spring fingers 24 are compressed to make a reasonably tight fit between the broad faces 6 of the waveguide 4. Similarly, the angled bend imparted to the narrow edge flanges 18 and the width of bulkhead 10 is arranged to cause these flanges to be compressed, the bulkhead fitting snugly between the narrow faces 8 of the waveguide.

The actual electrical function of providing a short or termination of the waveguide is performed by an element 26 made of conductive material formed into a squareshaped U. The base 28 of this element is normal to the longitudinal axis of the waveguide; the arms 30 are parallel to the broad faces 6 of the waveguide. No part of element 26 makes contact with the interior waveguide faces.

The base 28- of element 26 is maintained broadside to the direction of energy propagation and kept clear of they waveguide faces by support 32, a piece of conductive material having the edges turned up to form connecting flanges 34. The flanges 34 are welded to the inside of the base 28 of shorting element 26 and the front of base 20 of guide bulkhead 10; the distance from the shorting base 28 to guide base 20 being approximately a quarter wavelength at the operating frequency.

Attached to the back face of bulkhead base 20 is a vertically oriented U-shaped member designated 36. This too consists of a piece of rigid conductive stock bent to form a square U. A small portion of the tops of arms 38 of the member 36 are bent outwardly at right angles to form two attaching flanges 40 which are riveted or welded to base 20 of bulkhead 10. A portion of the edges of arms 38 which are adjacent the broad faces 6 of the waveguide 4 are bent inwardly at right angles forming four mounting flanges 42. Mounted on the outside of the four flanges 42 are strips 44 having a row of single contact spring fingers 46 formed therefrom. The vertical dimension of arms 38 corresponding to the narrow waveguide face 6 inner dimension is selected to cause the contact fingers 46 mounted thereon to be ap preciably compressed when inserted within a waveguide. These spring finger elements 46 as well as the fingers 24 on bulkhead 10 are kept clean by wiping against the interior faces of the waveguide; clean, certain contact is thereby assured at all times.

The broad dimension of the waveguide 4 in the back cavity is thus divided into three sections by the arms 38. Effectively the three sections become waveguide elements energizable only at frequencies for which the narrow dimension 8 of the waveguide 4 is a half wavelength or less. It sometimes occurs in high power systems that energy leaking past the shorting bulkhead 28 causes unexpected resonant oscillations in the back cavity with unpredictable ill effects on performance. The vertical plates 38 thus function to deny the back cavity to oscillations at the operating frequency.

The extremely low weight and small mass of the piston 2 permits a much simpler positioning means for moving the piston within its waveguide. A hole 48 in the center of the vertical base 50 of the U-shaped back element 36, and a corresponding hole 52 in the center of bulkhead base 10 receive any suitable positiot'ling means (not shown in the drawings).

Continued mechanical alignment of the shorting piston 2 is facilitated by the radically decreased mass and weight of the present invention compared with those used in the prior art-a decrease in the order of 800%. A typical piston used in WR975 waveguide weighed 8 pounds, a piston of the present invention slightly more than 1 pound. If holes 48 and 52 are in reasonable alignment with the usual aperture at the end of the waveguide where adjustment of piston location is made, the combination of snug bulkhead springing by fingers 18 and flanges 24 around all four sides of the waveguide, and the spring compression by fingers 46 on the two back plates 38 serves to keep the plane of the shorting bulkhead 10 always normal to the longitudinal axis of the waveguide. Thus, an appreciable external jarring of the waveguide system would impart a considerable mo mentum to the old style piston and tend to disturb both its location and the alignment of its shorting face, and even distort the waveguide faces. In contrast, the present invention has comparatively far less weight and mass than the pistons used hitherto, and less also than the section of guide which it contacts; being held off the faces of its waveguide by spring action it is relatively undisturbed by even appreciable external shocks to the waveguide.

Centain minor variations of this preferred embodiment will be apparent to those skilled in the art; and, therefore, it is not my intention to confine the invention to the precise form herein shown, but rather to limit it in terms of the appended claims.

Having thus described and disclosed a preferred embodiment of my invention, what I claim as new and desire to secure by Letters Patent of the United States is:

l. A tuning plunger for a rectangular waveguide com prising a sheet metal plate formed having edge flanges to fit within the internal dimensions of said waveguide, a strip of longitudinal spring contact elements secured around the outer surface of said flanges to make spring contact with the inner walls of said waveguide, a noncontacting quarter-wave short circuiting plunger extending into said waveguide from one surface of said plate and supported by said plate, a pair of spaced parallel sheet metal panels extending a substantial distance from the other surface of said plate and having edge flanges opposed to the inner broad walls of said waveguide, the spacing between said panels being such that they divide the broad dimension of said waveguide into three generally equal portions, and spring contact elements secured to said edge flanges on said panels for contacting said broad walls to electrically subdivide the volume of said waveguide and mechanically support the movable assembly in the Waveguide.

2. Apparatus according to claim 1 in which said short circuiting plunger comprises a sheet metal channel shaped member having a width slightly less than said broad dimension, the parallel portions of said channel being spaced slightly less than the narrow dimension of said waveguide and approximately a quarter wavelength long, and a plate extending from said one surface to the inner d surface of the bottom wall of said channel for supporting said channel.

3. A tuning plunger for a rectangular Waveguide comprising a mechanical sliding support formed from a plate transverse to said waveguide and two spaced parallel conductive panels projecting a substantial distance from one surface of said plate, said panels being parallel to the narrow Walls of said waveguide, sliding spring contacts on all edges of said plate and said panels which are opposed to an inner surface of the walls of said waveguide, and a non-contacting quarter Wave short circuiting plunger eX- tending into said Waveguide from the other surface of said plate, the extension of said panels being greater than required to effectively subdivide the Waveguide volume at the frequency of operation.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Ragan: Microwave Transmission Circuits, Radiation Laboratory Series, copyright 1948, pages 462 and 481- 483. 

